This invention relates, in general, to speed control systems for fluid driven prime movers; and, in particular, the present invention is directed to a prime mover speed control system based on a phase comparison between a variable reference input and feedback input during both a speed change and steady-state mode of operation.
Electro-hydraulic speed control systems for prime movers comprise an electronic comparator device for comparing an input reference signal, representing a desired prime mover speed, with a feedback signal, representing the actual prime mover speed, to obtain a speed error signal which is then applied to a valve operator which controls a prime mover inlet valve. The signals may either be analog signals having a voltage level proportional to speed or digital pulse trains having a frequency proportional to speed. One example of an analog electro-hydraulic prime mover control is formed in U.S. Pat. No. 3,097,488 issued July 16, 1963 to Eggenberger et al. In that patent, FIG. 9, taken in conjunction with the specification shows a reference speed signal having a voltage level proportional to a desired set speed being compared with a feedback speed signal having a voltage level proportional to actual prime mover speed to provide an error signal voltage. If the prime mover is operating at the reference speed, no speed error voltage will be generated. If the prime mover is operating at other than the reference speed an error voltage will be generated for actuating the valve operator. The initiation of valve actuation for speed error correction is dependent upon the occurrence of an actual speed error.
An example of a digital prime mover control is U.S. Pat. No. 3,798,907 to Barrett and Ahlgren issued Mar. 26, 1974. In that patent, a reference speed signal and a feedback speed signal are digital pulse trains each having a period which is inversely proportional to its frequency. The respective speed signals are input into a "comparator-integrator" which includes "period measuring logic," "error detection" by subtracting the reference period from the feedback period, "multiplication" and "accumulation." The patented control system is therefore based on an inverse frequency comparison or period comparison of digital signals and hence depends upon an actual speed error (frequency difference) between the reference signal and the feedback signal to provide for corrective valve actuation.
In U.S. Pat. No. 3,753,067 to Milligan a constant speed electric motor is regulated by a system in which either a direct drive signal or a digitally incremented biasing signal is applied to the motor to bring it up to speed or to maintain the motor speed under load. The object of the patented invention is to provide an electronic switching device which will maintain the constant speed motor at rated speed during incremental load changes by applying a relatively narrow bias voltage level rather than the full zero to maximum voltage range of the direct drive signal. In the implementation of this invention, a phase frequency comparator is used to compare a fixed reference speed digital pulse train with a motor speed feedback digital pulse train to provide a pulse signal output which increments the bias voltage level whenever the feedback signal lags the reference signal with respect to phase. Otherwise, the comparator output is zero. Hence, the patent describes a means for maintaining the steady-state speed of an electric motor once a reference frequency and the feedback frequency are synchronized by a direct drive signal.
It is one object of the present invention to provide a speed control system for a fluid driven prime mover which provides inlet valve adjustment proir to the occurrence of actual steady-state speed error.
It is another object of the present invention to provide a highly responsive speed control system for a prime mover adjustable over the full range of desired prime mover speeds.
The invention to be set forth in detail is a speed control system for use in combination with a fluid driven prime mover, such as a steam turbine, wherein the speed of the turbine is controlled by means of steam inlet valves. The steam inlet valves are hydraulically positioned against a spring biased closing force by means of a valve operator, for example, a servo valve. The servo valve is positioned by an output voltage from a phase-frequency detector. The phase-frequency detector receives two input signals both in the form of digital pulse trains. One input signal is a variable reference speed signal having a frequency proportional to the desired speed of the turbine. The other input signal is a feedback signal from the turbine itself having a frequency proportional to the actual turbine speed. The phase-frequency detector compares the two input signals and provides an output voltage which is linearly proportional to the phase difference between the reference speed pulses and the feedback speed pulses. The phase-frequency detector output also indicates whether the phase difference is a lead or lag and hence, whether the inlet valve should be moved to a more open or more closed position. Since an error voltage may be produced when the input phase relation changes, the turbine inlet valve may be adjusted prior to the occurrence of an actual steady-state speed error. Moreover, this variably adjustable speed control is highly responsive since during set speed changes an output signal immediately follows reference input signal change according to the phase change.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the drawings.